When drilling deviated or horizontal wells, conventional overbalanced drilling typically kills the well in older or lower production zones, making conventional logging techniques difficult to accomplish. Different techniques have been developed to circumvent this problem by using underbalanced techniques and sealing around the pipe and wireline cable for example thereby permitting gas insertion into the well bore annulus, logging while drilling (LWD), memory shuttles conveyed by pumping them down inside pipe or using slim tools such as RST conveyed with cable inside drill pipe. So far as known to applicant, the problems with conveying a full suite of open hole logging tools in an under-balanced horizontal well remained unsolved until the filing of applicant's companion application in the European Patent Office under Serial No. EP082901261 on or about 19 Mar. 2008, which application is incorporated herein by reference as if copied herein.
Horizontal wells are a commonly used technique in the industry. Horizontal wells enable a long interval of reservoir to be contacted in a single well thereby improving the productivity and enhancing reservoir recovery economics. Horizontal well logging techniques have also evolved. While drilling measurements allow accurate well placement, in certain types of reservoirs and depending on the evaluation objectives requested by the owner, there still remains a strong need to perform wireline logging operations involving high resolution imaging, magnetic resonance measurements, as well as downhole fluid analysis and sampling services—most of which are currently not available with LWD. For such cases, the industry has developed a method conveniently termed in current literature as “tough logging conditions” (TLC) which enables the tools to be conveyed on drill pipe while also maintaining an electrical connection to the surface unit using a standard wireline cable. The method, in summary, involves conveying tools in the well using drill pipe till just above the last casing shoe. A cable side door entry sub is then inserted in drill string to allow the cable to be rigged up and to enter inside the drill pipe through the side entry sub and further connect to the tools already down hole. The cable is tied up or fixed at the side entry sub and both cable and drill pipe are simultaneously conveyed down to perform logging operations. A standard feature of a TLC system is that a certain length of cable, equal to a minimum of the length of the logging interval, is carried on the drillstring outside the drillpipe located between the rig floor and the point in the drill string where the cable enters the drill pipe, that is, at the side entry sub.
Under-balanced drilling is especially suited for horizontal wells because formation damage in horizontal over-balanced wells can be very significant due to the long contact length and contact time between reservoir rocks and drilling fluids as well as constant scraping of filter cake by the drill pipe lying down on the low side of the horizontal. As a result, significant productivity is lost due to formation damage in horizontal wells. The industry has therefore realized the need to design technologies that are able to perform under-balanced drilling in horizontal wells to lower the formation damage that would otherwise be caused if the well was drilled over-balanced and thereby achieve higher productivity.
The main objective of this invention is therefore to provide an improved apparatus and methods for inserting a wireline through a blow out preventer stack, including either a rotating blow out preventer or snubbing preventer, to perform the equivalent of a TLC logging operation in an under-balanced horizontal well.